This invention relates to a method for monitoring patient respiration; more particularly, the invention relates to a method for monitoring patient respiration by evaluation of an electrical signal generated by an electrode coupled to the patient during respiration. The invention is primarily intended for respiration monitoring and analysis in furtherance of the detection and prediction of apnea.
The medical term "apnea" means cessation of respiration or breathing. The apneic condition has become associated in recent years with the "sudden infant death syndrome", wherein a disturbing pattern of early deaths of apparently healthy infants were noted. Studies and research programs have been conducted by agencies of the federal government and others in an attempt to identify the cause of sudden infant death syndrome, and such studies have revealed that infants who are subject to this syndrome are apparently not the healthy infants before death that they were once believed to be. These infants appear to have subtle and anatomic and physiological defects of a neurologic, cardiorespiratory and/or metabolic nature. There has developed evidence that the syndrome is not caused by a single mechanism working at one moment in time, but rather by a number of developmental, environmental, and pathologic factors which become involved in complex interactions and circumstances to set up a sequence of events that produces the sudden, unexpected and unexplained infant death. Much of this research has revolved around the hypothesis that apnea during sleep is related to the syndrome. It has also been postulated that apneic episodes during sleep which do not necessarily lead to sudden infant death may lead to aberrations in central nervous system development. There is evidence that with infants having numerous apneic episodes during sleep, the resulting inadequate oxygen supply to the brain may lead to retardation of brain development, which in turn may lead to further loss of respiratory control and further apneic problems.
Apnea may be caused by a number of other factors not necessarily related to conditions of infants, some of which are spinal cord injury, muscular dystrophy, lung diseases, drug intoxication, and certain other risk factors which have become apparent in aid of the identification of those who might be candidates or may suffer higher than normal risk for incurring apneic events. In adults, a history of heavy snoring denotes an individual at risk to apnea, especially in combination with other such factors as obesity, underlying heart desease and/or high blood pressure.
Whereas apneic conditions may be monitored in hospital and laboratory environments by means of suitably connected electrodes to a patient's body, and monitoring of a cathode ray tube (CRT) display which exhibits a wave form related to patient respiration, such monitoring is impractical or impossible in less controlled environments. There is therefore a need for techniques and devices which will enable appropriate monitoring of adults or infants and which will detect apneic conditions in time to set off an alarm in order that the condition may be corrected. For sleeping patients, it is frequently only necessary to wake the patient by means of an audible alarm or other alarm indication in order that the patient may become conscious and resume normal breathing. It is therefore important that techniques and devices be developed for utilization in conjunction with sleeping individuals, in order that apneic episodes may be detected and corrected before physiological damage or harm occurs. In all events, the detection of apnea requires a monitoring of respiration and/or heart rates. Such monitoring is accomplished by means of the attachment or coupling of suitable electrodes or other transducer devices to the patient, so that signals developed by such devices may be transmitted to circuitry for detection and analysis. Monitoring transducers are well-known in the art, as for example the respiration monitoring apparatus described in U.S. Pat. No. 3,760,794, issued Sept. 25, 1973. This device is a capacitor transducer constructed from alternate layers of conductive and nonconductive materials which are placed beneath a patient in a mattress or other resilient support, and which detects motion caused by respiration. In other devices, electrodes are attached to the body to detect changes in body impedance or resistance as respiration occurs. In still other devices, pressure sensing transducers are used in the chest and/or stomach area of the body to detect pressure changes which occur as body movement occurs during respiration. All of these transducers, as well as other similar devices, monitor a physical parameter which is correlated with respiration, convert the parameter monitored into an equivalent electrical signal, and feed this signal into appropriate circuitry whose ultimate function is to note the occurance of a respiration cycle and to perform such other functions as are deemed necessary.